Different cable attenuation occurs in antenna systems of radio microphones. This must be considered, in which case amplification control in the antenna is required to compensate cable attenuation by means of active components in order to maintain the optimal possible combination of intercept and noise factor. Since the cables in antenna systems have frequency-dependent attenuation, an optimal combination of intercept and noise factor is only possible at one frequency point in a broadband amplifier, whether it is controllable or constantly amplified. To this it must be added that the slope of frequency-dependent attenuation changes with cable length so that there is an additional parameter to be considered.
This so-called slope compensation plays a role in the field of telecable technology and satellite technology, U.S. Pat. No. 4,967,169 (“the '169 patent”) and U.S. Pat. No. 5,144,267 (“the '267 patent”) being referred to in this context. The '267 patent discloses a compensation circuit with controlled resistances, based on a set of diodes. One series tuned circuit provides for varying degrees of negative slope compensation and a parallel tuned circuit provides for varying degrees of positive slope compensation. The network elements are connected in a modified bridge T configuration. These controllable attenuators generally have an adverse effect on the intercept (i.e., large signal behavior).
However, the problem underlying the invention in the field of radio microphones of selecting amplification of the active element so that subsequent cable attenuation is always compensated (i.e., without worsen the intercept) cannot be solved. This problem is again due to the fact that, depending on the type of cable employed and the employed cable length, the cable attenuation is different, for which reason it is always necessary in the prior art to provide non-constant, but variable amplification of the active elements. This occurs, for example, by providing a controllable attenuator in series with a radio frequency (“RF”) cable so that the combination of the amplifier, the RF cable and the controllable attenuator is set at 0 dB. It would also be possible to make the gain of the amplifier variable, which, however, generally has an adverse effect on noise factor and/or intercept.
It is naturally a basic requirement that amplification is greater than attenuation of the employed RF cable. The optimization of the ratio or combination of intercept and noise factor just described at one frequency point is therefore possible. This means that a change of 0 dB occurs at this optimal frequency point, whereas at lower frequencies, at which cable attenuation is lower, amplification occurs at the end of the chain and at higher frequencies, at which the cable attenuation is higher, lower amplification occurs, or, when the frequency band center is set at 0 dB, attenuation is actually present for the uppermost frequencies and the noise factor is therefore also increased.
Japanese Patent Publication JP 2007/281735 deals with the problem to prevent interference between adjacent channels by controlling signal attenuation of a desired channel in a system installation. This is achieved by controlling the attenuators in dependency of the output of a high frequency mixing distributor. Again, these controllable attenuators generally have an adverse effect on noise factor and/or intercept.